1. Field of the Invention
The present invention relates to a power source circuit, and more particularly, to a power source circuit for converting a low-frequency alternating current into a high-frequency alternating current via a direct current and outputting the converted current.
2. Description of the Related Art
Recently, regulations for a high frequency with respect to an input current of a power source circuit, such as an inverter for illumination to be connected to a commercial AC power source, have become more strict year after year. For this reason, a circuit must realize not only a high efficiency, as in a conventional circuit, but also a low-distortion input. In a known conventional arrangement, a chopper circuit is provided at an input to control an input current, thereby realizing such a low-distortion input.
When a chopper circuit is provided at an input, however, a two-stage conversion of the chopper circuit and an invertor is arranged in a power source circuit to increase the number of constituting parts. Therefore, an inexpensive power source circuit cannot be obtained.
For this reason, arrangements disclosed in Unexamined Published Japanese Patent Application No. 61-94569 and No. 2-202365 are known. For example, in the circuit disclosed in Patent 61-94569, as shown in FIG. 1 (PRIOR ART), a rectifier 14 is connected to the output terminal of a commercial AC power source 12. One terminal of a reactor 16 is connected to one output terminal of the rectifier 14, and a smoothing electrolytic capacitor 18 is connected to the other terminal of the reactor 16. A series circuit constituted by reverse-conductive switches 20 and 22 and a series circuit constituted by two capacitors 24 and 26 are connected in parallel with the two terminals of the electrolytic capacitor 18. A node between the switches 20 and 22 is connected to the other terminal of the rectifier 14. A load 28 is connected between a node between the switches 20 and 22 and a node between the capacitors 24 and 26.
In the power source circuit having the above arrangement, the reverse-conductive switches 20 and 22 are alternately switched on/off at a high frequency. As a result, the circuit operates as an inverter, and the switch 20 operates as a chopper to charge the smoothing capacitor 18.
In the above conventional circuit, however, since boosting is performed by the chopper operation, a DC voltage must be designed to be at least two time more than a peak voltage of an AC voltage. If an AC voltage is 100 V, a corresponding DC voltage is about 300 V, and no problem is posed in selection of a switching element. If an AC voltage is 200 V, however, a corresponding DC voltage is 600 V, and it is difficult to select a switching element.
In addition, since an input current is intermittently supplied by the chopper operation, a filter circuit is required to perform smoothing. For this reason, a large capacitance and a large inductance are required.
Furthermore, since the reverse-conductive switch 20 singly performs switching of all the capacitance, the switch 20 must have a large rated current. For these reasons, manufacturing cost of the circuit is increased.